Cabletron Systems BRIM-F6 manual Figure A-4. Token and Frame Formats

APPENDIX A: BASIC FDDI NETWORKS

Other functions of the PHY include generation of a 125 Mhz transmit clock, synchronization of the receive clock with an upstream transmitter, encoding and decoding for media control symbols, and in some applications, buffering for the incoming bit stream.

The MAC entity resides on the lower sublayer of the Data Link layer. The upper sublayer, Logical Link Control (LLC) serves as an interface between the OSI model and the FDDI network. The MAC element, under control of Station Management, performs many of the tasks associated with frame preparation and media access: ring scheduling, token generation, timers that monitor ring activity, ring initialization, and beaconing. Other tasks for the MAC entity include assembling data frames, maintaining medium addressing, and generating and checking data check bytes.

The MAC generates two basic message formats, tokens and frames. Figure A-4 shows the layout for each message. Control and format bits within the header define specific types of frames and classes of tokens. The MAC sublayer receives transmittable Data frames from the LLC as Service Data Units (SDUs). MAC uses these SDUs to construct Protocol Data Units (PDUs) that it passes on to PHY. The PDUs consist of a MAC header, the encapsulated SDU, and a Frame Check Sequence (FCS). The MAC generates FCS during transmission.

TOKEN

Preamble

≥16 Symbols

Starting Frame Ending

Delimiter Control Delimiter

2 Symbols 2 Symbols 2 Symbols

JK	TT	Frame Check	T
		Sequence Coverage

FRAME

Preamble

≥16 Symbols

Starting Frame

Delimiter Control

2 Symbols 2 Symbols

Destination Address	Source Address	Information
4 or 12 Symbols	4 or 12 Symbols	≥ 0 Symbol Pairs

Frame Check

Sequence

8 Symbols

Ending

Delimiter

1 Symbols

Frame Status

≥3 Symbols

Maximum - 9000 Symbols

Figure A-4. Token and Frame Formats

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